This invention relates in general to a method for increasing the operational efficiency of a fuel cell power plant, and deals more particularly with a method of providing a fuel cell power plant with a reactant stream which increases the utilization of the reactant stream, thereby increasing the performance of the fuel cell power plant as a whole.
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a reactant fuel being provided to an anode electrode and a reactant oxidant being provided to a cathode electrode, generating an external current flow therebetween. Such fuel cell assemblies are very useful due to their high efficiency, as compared to internal combustion fuel systems and the like, and may be applied in many fields. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products, typically water, which are produced during their operation. Owing to these characteristics, amongst others, fuel cell assemblies are particularly applicable in those fields requiring highly reliable, stand-alone power generation, such as is required in space vehicles and mobile units including generators and motorized vehicles.
Typically, electrochemical fuel cell assemblies employ a hydrogen-rich gas stream as a fuel and an oxygen-rich gas stream as an oxidant, whereby the resultant reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the anode and cathode electrodes formed of porous, electrically conductive sheet materialxe2x80x94typically, carbon fiber paper. One particular type of ion exchange membrane is known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION(trademark) and well known in the art. Catalyst layers are formed between the PEM and each electrode to promote the desired electrochemical reaction. The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly.
In the typical operation of a PEM fuel cell assembly, a hydrogen rich fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the PEM to the cathode electrode while the electrons flow through an external circuit connected to a load. At the cathode electrode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode electrode at the catalyst layer to form the by-product water. Not only does the PEM facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen rich fuel from the oxygencontaining gas oxidant. The reactions taking place at the anode and cathode catalyst layers may be represented by the following equations:
Anode reaction H2xe2x86x922H++2e
Cathode reaction: 1/2O2+2H++2exe2x86x92H2O
In practical applications, a plurality of planar fuel cell assemblies are typically arranged in a stack, commonly referred to as a cell stack assembly. The cell stack assembly may be surrounded by an electrically insulating housing that defines the various manifolds necessary for directing the flow of a hydrogen-rich fuel and an oxygen-rich oxidant to the individual fuel cell assemblies, as well as a coolant stream, in a manner well known in the art. A fuel cell power plant may typically be comprised of the fuel cell stack assembly, reactant storage vessels, reactant control valves, reactant propulsion devices, coolant pumps, heat exchangers, coolant degassifiers or demineralizers, sensors for measuring reactant concentrations, temperatures, pressures, current, voltage, and a microprocessor that controls the operation of the fuel cell power plant.
As will be appreciated by one so skilled in the art, tying these differing components into a cohesive fuel cell power plant operating within specific design parameters results in a complex and oftentimes cumbersome structure.
Specifically, the operating efficiency of a fuel cell power plant is directly related to the utilization of the reactant fuel stream supplied to the fuel cell assemblies making up the fuel cell power plant. This utilization, commonly referred to as xe2x80x98hydrogen utilizationxe2x80x99, due to the use of a hydrogen-rich fuel stream in PEM fuel cells, is the ratio of reactant fuel consumed at the anode electrode of the fuel cells, divided by the total quantity of reactant fuel supplied to the fuel cells multiplied by 100. While PEM fuel cell power plants are designed to come as close as possible to 100% utilization, this is practically unfeasible.
Current generation PEM fuel cells frequently use thin polymer membranes on the order of approximately 15 microns thick to maximize cell performance of approximately 1000 amps per square foot (ASF). A certain measure of the hydrogen utilized as fuel within these fuel cells will diffuse across this thin membrane from the anode electrode to the cathode electrode to react with the oxygen-rich oxidant to form water. Likewise, oxygen also tends to diffuse across this thin membrane to combine with hydrogen to form water.
The combination of these effects is to reduce the available hydrogen capable of reacting at the anode electrode of the fuel cells and hence, reduces the utilization rate of the fuel cell power plant as a whole.
It is common for PEM fuel cell power plants to be operated by supplying a reactant fuel to the integrated cell stack assembly using a cascade or multiple-pass approach, wherein the individual fuel cells in the cell stack assembly are separated in two or more groups. The reactant fuel is supplied to the first group of fuel cells and then cascades to the next group and so on until exiting the cell stack assembly through a fuel exit manifold. With such an arrangement, the practical hydrogen utilization has been found to be as high as 90% or more for the overall cell stack assembly, with individual cascade groups operating at approximately 60-70% utilization. Many different flow orientations have, however, been utilized in fuel cells. The objective is always to distribute the hydrogen flow such that every section of the fuel cell receives the required quantity of fuel. Flow configurations that have been used within the cell are single pass, two pass, multi-passes, serpentine and interdigitated. Other times an external recycle is used between the cell exit and the cell inlet to improve flow uniformity within the cell stack assembly.
It has been observed, however, that in certain circumstances, for instance during shutdown or startup of the PEM fuel cell power plant, some cascade groups may suffer from fuel starvation to such an extent that the fuel cell power plant is unable to achieve a desired power output. The fuel starvation is typically caused by the latter cascade groups being momentarily deprived of the fuel stream while it is cascading from the first cascade group to the last. In addition, fuel starvation may also occur during ongoing operation of the fuel cell power plant due to nitrogen contamination of the fuel stream, this nitrogen is typically present in the reactant oxidant stream that is also being supplied to the cell stack assembly. In many applications the oxidant stream is comprised of atmospheric oxygen which contains a nitrogen component that diffuses, in part, through the solid polymer PEM to dilute the hydrogen-rich fuel stream. The diluted hydrogen fuel stream may therefore not contain an adequate hydrogen concentration to support the required current density in some sections of the cell stack assembly, thereby corroding the anode electrode in some of the fuel cell assemblies and possibly leading to catastrophic damage.
It has previously been proposed to eliminate the cascade architecture in favor of a single-pass arrangement wherein each fuel cell in the cell stack assembly is provided with the reactant fuel stream via a common feed manifold while increasing the vent rate; however, this results in a lowering of the hydrogen utilization to approximately 80%, an unacceptably low efficiency rate for most applications. The phrase xe2x80x98vent ratexe2x80x99 as utilized above and hereinafter represents the rate at which the hydrogen fuel stream is exiting the anode electrode, that is, the anode flow fields, of a fuel cell, or cell stack assembly.
It has also been known to address the concern over impurities in both the reactant fuel and oxidant streams by totally purging the reactant flow chambers of a cell stack assembly, as practiced by the Assignee of the present invention in conjunction with cell stack assemblies provided to NASA for the space shuttle program. It will be readily appreciated that the complete purging of both the fuel and oxidant reactant streams is economically unadvantageous and therefore inappropriate when dealing with cell stack assemblies utilizing an oxidant reactant comprised of an non-pure oxygen stream in contact with a relatively thin PEM, as opposed to the pure oxygen stream and comparatively large PEM utilized with the cell stack assemblies manufactured for NASA shuttle program and the like.
With the forgoing problems and concerns in mind, the present invention therefore seeks to increase the hydrogen utilization for a PEM fuel cell power plant while simultaneously preventing fuel cell starvation and excessive dilution of the reactant fuel stream.
It is an object of the present invention to increase the operational efficiency of a fuel cell power plant.
It is another object of the present invention to increase the hydrogen utilization of a fuel cell power plant.
It is another object of the present invention to increase the hydrogen utilization of a fuel cell power plant while eliminating fuel cell starvation.
It is another object of the present invention to increase the hydrogen utilization of a fuel cell power plant while reducing the dilution of the reactant fuel stream.
According to one embodiment of the present invention, a method is proposed for increasing the operational efficiency of a fuel cell power plant including a cell stack assembly comprised of a plurality of fuel cells in electrical communication with one another. The cell stack assembly includes a fuel inlet manifold and a fuel exit manifold for accepting and exhausting, respectively, a reactant fuel stream. The proposed method includes providing the cell stack assembly with the reactant fuel stream, sealing the fuel exhaust manifold for a first predetermined time period, thereby preventing the reactant fuel stream from exiting the cell stack assembly and opening the fuel exhaust manifold for a second predetermined time period.
These and other objectives of the present invention, and their preferred embodiments, shall become clear by consideration of the specification, claims and drawings taken as a whole.